Rubber, abrasion resistance Physical properties

Natural rubber Solid Good physical properties and resistance to cutting and abrasion. Low heat and ozone resistance. Gaskets. 

Vulcanization changes the physical properties of rubbers. It increases viscosity, hardness, modulus, tensile strength, abrasion resistance, and decreases elongation at break, compression set and solubility in solvents. All those changes, except tensile strength, are proportional to the degree of cross-linking (number of crosslinks) in the rubber network. On the other hand, rubbers differ in their ease of vulcanization. Since cross-links form next to carbon-carbon double bonds. 

Synthetic rubbers (elastomers) are long-chain polymers with special chemical and physical as well as mechanical properties. These materials have chemical stability, high abrasion resistance, strength, and good dimensional stability. Many of these properties are imparted to the... 

The carboxylated types (XNBR) contain one, or more, acrylic type of acid as a terpolymer, the resultant chain being similar to nitrile except for the presence of carboxyl groups which occur about every 100 to 200 carbon atoms. This modification gives the polymer vastly improved abrasion resistance, higher hardness, higher tensile and tear strength, better low temperature brittleness, and better retention of physical properties after hot-oil and air ageing when compared to ordinary nitrile rubber. 

The retention of the maximum tensile strength at elevated temperatures is greater for radiation cured than for chemically cured natural rubber. The physical properties after high-temperature aging are not improved, however. Lower flex life and higher abrasion resistance of radiation cross-linked NR were reported. Other properties, such as permanent set, hardness, and resilience, were found to be nearly equal. 

The physical properties of natural rubber and synthetic rubber compounds are affected greatly by the type and amount of fillers used. Carbon black is the most commonly used filler. Increasing amounts of carbon black increases the hardness and modulus of the vulcanizates. Resilience and resistance to impinging type abrasion decrease along with elongation. Tensile strength and tear strength... 

The hard, tough metals or elastomeric polymers may be used to resist the cavitation erosion. The physical property intervening here is the resilience, which is the capacity to dispose the energy of an impact without absorbing some. Another property of the rubber that can also have an influence on the resistance to cavitation erosion is its resistance to abrasion.25... 

The outstanding property of rubber in general is resilience, or low modulus of elasticity. The flexibility and physical properties of rubber account for its application in general engineering and automobile industries whereas its chemical, wear and abrasion resistance as a sacrificial material, plus its insulating properties are utilised in many corrosion and erosion applications in process industries. Rubber lined mild steel, pipes and tankages have been standard materials of construction for hydrochloric acid service for many years. 

Fillers are added to the elastomer in order to add bulk, lower cost and/or to improve physical properties such as hardness, strength and abrasion resistance. Typical fillers are materials such as carbon black, talc, china clay and whiting. Carbon black has been shown to contain polynuclear aromatics (PNAs) and there is concern regarding their carcinogenicity (Lee and Hites, 1976). However, despite extra controls there has been a move away from the use of carbon black as a filler in applications involving the primary packaging of parenterals. Its use continues as a pigment or colourant in rubber formulations but at substantially lower levels than that as a filler. 

Table 15-11 gives the physical properties of a urethane elastomer compared with cured natural and synthetic rubbers. One outstanding property of urethane elastomers is resistance to abrasion. 

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

Polyurethane is a unique material that offers the elasticity of rubber combined with the toughness and durability of metal. Because urethane is available in a very broad range of hardness (soft as an eraser to hard as a bowling ball), it allows the engineer to replace rubber, plastic, and metal with the ultimate in abrasion resistance and physical properties. 

PROPERTIES OF SPECIAL INTEREST Standard emulsion SBR is a general purpose rubber. Most widely used synthetic rubber in the world. Better tire tread-wear and aging properties than natural rubber. Good abrasion resistance and crack initiation resistance. Poor in tack and heat build-up. Physical properties are poor without reinforcing fillers. Solution SBR is a speciality rubber and more expensive than emulsion SBR. Solution SBR with high vinyl and styrene levels is used in high performance tire treads to improve wet traction. Also used as impact modifier in plastics and as thermoplastic elastomers. 

Thermoplastic polyurethane elastomers are polymers that bridge the gap between rubbers and plastics. They can be used in a wide range of properties, from hard rubbers to soft engineering thermoplastics as they are elastic and melt-processable. They can be processed on extrusion as well as injection, blow and compression molding equipment. They can be vacuum-formed or solution-coated and are suited for a wide variety of fabrication methodologies. They provide a considerable number of physical property combinations high resilience, good compression set, resistance to abrasions, tears, impacts, weather, and even hydrocarbons. Such materials... 

Nitrile and Acrylic Rubber. Nitrile rubbers are made by the emulsion copolymerization of acrylonitrile (9-50%) and butadiene (21) and are abbreviated NBR (eq. 11). The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties and the nature of the pol5nners. As the ACN content increases, the oil resistance of the poljnner increases (14). As the butadiene content increases, the low temperature properties of the polymer are improved. Nitrile rubber is much like SBR in its physical properties. It can be compoimded for physical strength and abrasion resistance using traditional fillers such as carbon black, silica, and reinforcing clays. The primary benefit of the polymer is its oil and solvent resistance. At a medium ACN content of 34% the volume swell in IRM 903 oil at 70°C is typically 25-30%. Nitrile rubber can be processed on conventional rubber equipment and can be compression, transfer, or injection molded. It can also be extruded easily. Nitrile rubber compoimds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from -30 to 125°C. The compounds are also plasticized nsing polar ester plasticizers. 

The single outstanding physical property of butyl rubber is its impermeability. Its abrasion resistance, tear resistance, and adhesion to fabrics and metals is good. The flame resistance of butyl rubber is poor. Table 4.7 lists the physical and mechanical properties of butyl rubber. 

This elastomer also exhibits good resistance to impact, abrasion, tearing, and cut growth over a wide temperature range. The physical and mechanical properties of ethylene-propylene rubber can be found in Table 4.14. Ethylene propylene terpolymer has similar physical properties. 

The physical and mechanical properties of the nitrile rubbers are very similar to those of natural rubber. Buna-N does not have exceptional heat resistance. It has a maximum operating temperature of 200°F/93°C and has a tendency to harden at elevated temperatures. The nitrile rubbers will support combustion and burn. NBR has good abrasion resistance and tensile strength. 

Polymer alloys are a commercial polymer blend with improvement in property balance with the use of compatibilizers. They exhibit an interface and show varied physical characteristics. Sometimes they have excellent physical properties in one area but possess poor physical properties in others. For example, silicone rubber has poor oil and abrasion resistance but possess excellent heat resistance. A product solution in this regard would be to obtain a polymer blend with constituents possessing physical properties that complement each other such that the resultant polymer blend would exhibit superior physical properties compared with the components of the blend. 

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